dementia, general term for any of a number of conditions marked by chronic, usually progressive deterioration of intellectual capacity associated with the widespread loss of nerve cells and the shrinkage of brain tissue. People with dementia experience impairments in memory and learning, executive functioning, complex attention, language, perceptual-motor or visuospatial function, and social cognition. It is most commonly seen in the elderly (senile dementia), but dementia is not an inevitable part of the aging process and can affect persons of any age. In 2005 researchers reported that some 24.3 million people worldwide were living with dementia. In 2015 this figure rose to an estimated 47.5 million, a number that was expected to increase significantly by 2030, in part because of anticipated increases in life expectancy in many countries. In the 2013 revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM), there was a shift from the term dementia, and this group of disorders was categorized under “major neurocognitive disorder” in DSM-5. For a major neurocognitive disorder diagnosis, a patient must experience cognitive impairment that significantly interferes with everyday functioning. This impairment should be evident when compared with one’s previous levels of cognitive functioning. For less severe forms of cognitive impairment, DSM-5 uses a “mild neurocognitive disorder” diagnosis. The term dementia, however, continues to be in use in society and medical literature.

The most common irreversible dementia is Alzheimer disease. This condition often begins with memory loss or with subtle impairments in other cognitive functions. These changes may manifest initially as simple absentmindedness or forgetfulness or as minor problems with judgment, language, or perception. As the disease progresses, memory loss and cognitive impairment broaden in scope until the individual can no longer remember basic social and survival skills or function independently. Language, spatial or temporal orientation, judgment, perception, and other cognitive capacities decline, and personality changes may occur.

Dementia is also present in other degenerative brain diseases, including Pick disease and Parkinson disease. Another form of dementia, known as limbic-predominant age-related TDP-43 encephalopathy (LATE), closely resembles Alzheimer disease. Although LATE is also marked by the deterioration of memory and cognition and declines in social skills, patterns of neurocognitive change and the rate of decline in LATE differ from Alzheimer disease. Other major forms of dementia are dementia due to Lewy body disease, frontotemporal dementia, and dementia due to psychoactive substances.

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The second most common cause of dementia is hypertension (high blood pressure) or other vascular conditions. This type of dementia, called multi-infarct, or vascular, dementia, results from a series of small strokes that progressively destroy the brain. Dementia can also be caused by Huntington disease, syphilis, multiple sclerosis, acquired immune deficiency syndrome (AIDS), and some types of encephalitis. Treatable dementias occur in hypothyroidism, other metabolic diseases, and some malignant tumors. Treatment of underlying disease may inhibit the progress of dementia but usually does not reverse it.

Because many instances of dementia are not necessarily a consequence of aging but rather are associated with lifestyle factors, certain behaviors can help delay or potentially prevent dementia. Guidelines for dementia prevention issued by the World Health Organization include engaging in physical activity, never smoking, limiting alcohol intake, eating a healthy diet, and managing weight. Particularly in persons age 55 or older, risk of dementia may be increased by the use of certain prescription drugs, especially anticholinergic agents, which are used to treat a variety of conditions, including allergies, bladder disorders, chronic obstructive pulmonary disease (COPD), and depression.